1. Field of the Invention
This invention relates to a semiconductor optical device to be suitably used as for optical telecommunications, optical instrumentation, optical data processing and other areas of application and, more particularly, it relates to a semiconductor laser device that effectively operates in an oscillation wavelength band from 0.6 to 2.0 .mu.m.
2. Prior Art
It is critically important for a semiconductor laser device to have a reduced threshold current (threshold current density) for laser oscillation in order to operate stably and continuously with an enhanced output level. A technique currently used to reduce the threshold current density is to provide a large optical confinement coefficient relative to the active layer.
FIGS. 10 through 12 respectively illustrate the energy band diagrams for conductive band for known semiconductor laser devices of three different types.
FIG. 10 shows the conduction band of a multiquantum well type semiconductor laser device, which comprises an MQW (Multi Quantum Well) active layer 1, upper and lower optical confinement layers 2 and 3 sandwiching the active layer 1, an upper p-clad layer 5 and a lower n-clad layer 4 arranged respectively on the upper optical confinement layer 3 and under the lower optical confinement layer 2 to form an SCH (Separate Confinement Hetero) structure.
An SCH structure can provide a large optical confinement coefficient for the MQW active layer 1 and reduce the threshold current.
The multiquantum well semiconductor laser device illustrated in FIG. 11 comprises superlattice optical confinement layers 2 and 3 in the form of equiperiodically arranged thin layers in order to raise the potential height against electrons in the quantum walls while maintaining the optical confinement coefficient to the same level as that of conventional SCH.
As the quantum wells of the superlattice optical confinement layer of the semiconductor laser device of FIG. 11 have a reduced well thickness, it shows a high confinement energy for electrons there, which may be eventually be cut off to fall into a condition where no quantum state exists.
The multiquantum well semiconductor laser device shown in FIG. 12 additionally comprises a MQB (Mutli Quantum Barrier) 6 inserted between the optical confinement layer 3 and the p-clad layer 5.
The semiconductor laser device of FIG. 12 suppresses any overflow of carriers from the optical confinement layer into the upper clad layer (p-clad layer) with the high barrier effect produced in the MQB 6 due to the interference effect of electron waves.